1. Field of the Invention
This invention relates to a novel liquid coating composition suitable for use in forming photoresist coating films, as well as a photoresist material using said composition. More specifically, the invention relates to a liquid coating composition for use in the photolithographic formation of a photoresist pattern to form an anti-interference film on a photoresist film which is capable of sufficiently reducing the multiple interference of light in the photo-resist film to lessen the chance of degradation in the dimensional precision of the photoresist pattern. The invention also relates to a photoresist material of a dual structure that comprises a photoresist film overlaid with an anti-interference film formed of said liquid coating composition.
2. Description of Relevant Art
Photolithography as employed in the fabrication of semiconductor devices comprises the steps of providing a photoresist film on a substrate such as a silicon wafer, selectively exposing it by illumination with an actinic radiation such as UV light, far UV light, excimer laser, X-rays or electron beams, and performing development to form a photoresist pattern on the substrate. Photo-resists are of two types. A negative-working photoresist is such that the unilluminated area is dissolved away during development, while a positive-working photoresist has the illuminated area dissolved away. Either type of photoresists are selectively used as appropriate for a specific object.
With the recent improvement in the degree of integration of semiconductor devices, R & D efforts are being directed to production equipment adapted for microelectronic fabrication technology. Under the circumstances, the use of exposing equipment that operates on single-wavelength actinic radiations such as g-line, i-line and excimer laser is increasing because of its compatibility with microelectronic fabrication technology.
In the formation of resist patterns by photolithography, incident light is known to undergo multiple interference in the photoresist film and this causes variations in the linewidth of resist patterns with varying thicknesses of the photoresist film. The cause of this multiple interference effect of light is as follows: illuminating light falling at a single wavelength on the photoresist film interferes with the reflected light from the underlying substrate so that the absorption of light energy varies with the thickness of the photoresist film. Hence, variations in the thickness of the photoresist film will adversely affect the linewidth of the developed resist pattern and eventually degrade its dimensional precision. When forming a fine pattern on a substrate having steps, this phenomenon presents an unavoidable serious problem since the thickness of the photoresist film inevitably differs at different heights of each step. Hence, it is desired to develop a technology that eliminates the multiple interference lithography, effect of light to insure that the dimensional precision of fine patterns will not be degraded even if they are formed on a substrate having steps.
It has heretofore been proposed that the multiple interference effect of light be lessened by forming an anti-reflective (anti-interference) film on a surface of a substrate (see U.S. Pat. No. 4,910,122) or by first forming a photoresist film on the substrate and then forming a film of a water-soluble resin (e. g. polysiloxane or polyvinyl alcohol) as an antireflective film over the photoresist film (see Japanese Patent Publication (KOKOKU) 4-55323, Japanese Patent Public Disclosure (KOKAI) 3-222409, etc.). The first approach is effective to some extent in lessening the multiple interference effect of light; however, if light having the same wavelength as exposing light is used in mask alignment, the antireflective film will attenuate the alignment detection signal, making it difficult to achive high alignment accuracy. Additionally, the resist has to be transferred patternwise and precisely onto the antireflective film, which must thereafter be etched or otherwise stripped away without affecting the device being fabricated. Since this increases unavoidably the number of process steps involved, the first approach which comprises forming the antireflective film on a surface of the substrate is not applicable to all cases of substrate processing. On the other hand, the second approach, which comprises forming an antireflective film on a photoresist film over the substrate, is practical since it does not require an intricate process. However, none of the anti-reflective films formed from the conventional liquid coating compositions have proved to be completely satisfactory in their anti-interference action. The slightest interference effect can influence significantly the dimensional precision of photoresist patterns if they are very fine and, hence, the anti-interference films available today are incapable of meeting the recent demand for smaller feature sizes in the fabrication of semiconductor devices. Under the circumstances, there is a strong need to develop a more effective anti-interference film.
Anti-interference films are conventionally formed by spin coating or otherwise applying liquid coating compositions onto the photoresist film and in order to prevent the corrosion of applicators and, related equipment, it has been desired to develop a liquid coating composition that permits final pH adjustment to neutrality. In this regard, EP 0522990 A1 has proposed an anti-reflective coating that comprises an aqueous processable, film-forming fluorine-containing composition. The patent teaches that a fluoro-carbon compound can be contained in the fluorine-containing composition and specific examples of this compound include perfluoroalkyl acids, perfluoroalkyl sulfonic acids, etc. in the form of salts such as ammonium and tetramethyl ammonium salts. However, liquid coating compositions containing these compounds are acidic (with pHs of about 2-4) and, hence, there is high likelihood that applicators and related equipment are corroded by acid. Attempts to bring the liquid coating composition to neutrality have been unsuccessful since those salts of fluorocarbon compounds either gel or become completely insoluble to form a precipitate. Hence, it is desired to develop a liquid coating compositions for forming Photoresist coating films that serve satisfactorily as antireflective films and that yet are capable of preventing the corrosion of applicators and related equipment.
For the purpose of lessening the chance of degradation in the dimensional precision of Photoresist patterns, reducing the multiple interference effect of light in the Photoresist film by coating it with an anti-interference film is not sufficient and it is also necessary that the anti-interference film be applied uniformly to eliminate any unevenness in deposit. In most cases, conventional liquid coating compositions that use a water-soluble, film-forming component and a fluorosurfactant are in effect composed of these two components only. However, if such two-component liquid coating compositions are applied onto photoresists to form anti-interference films, the resulting coating is poor in the uniformity of application and there is high likelihood for unevenness in deposit. If any uneven deposit occurs, the resulting resist pattern will be interrupted in the defective areas, thereby rendering it impossible to produce a resist pattern in good registry with the mask pattern.
The present inventors conducted intensive studies with a view to solving these problems of the prior art and found that their objective could be attained by either a liquid coating composition containing both a water-soluble, film-forming component and a fluorosurfactant having a specified chemical structure or a liquid coating composition containing said water-soluble, film-forming component, an unspecified fluorosurfactant, and an anionic surfactant having a specified chemical structure. Hence, by using a photoresist material of a dual structure that comprises a photoresist film overlaid with an anti-interference film formed from either of those liquid coating compositions, one can form a satisfactory resist pattern having high fidelity to a mask pattern.